(Adds number of people missing down to 13)
By Jonathan Kaminsky
DARRINGTON, Wash., April 2 (Reuters) - Recovery teams raced on Wednesday against a forecast of returning showers in their search for victims of a Washington state mudslide that obliterated a community last month and left dozens dead or missing.
Hundreds of search-and-recovery workers have redoubled efforts since Monday to make the most of a three-day dry spell and receding waters after a week of heavy downpours that turned the disaster site in the foothills of the Cascade Mountains into a virtual swamp.
The official death toll rose to 29, based on victims' remains received by coroners, from 28 on Tuesday, the Snohomish County Medical Examiner's office said. Thirteen people were listed as missing, down from 22 a day earlier, said incident command spokeswoman Sheri Badger.
On March 22, a rain-soaked hillside collapsed without warning above the north fork of Stillaguamish River, unleashing a torrent of mud that roared over the river and across state Highway 530, engulfing some three dozen homes on the outskirts of the town of Oso.
"This is a community that they lost everything," said local fire department Lieutenant Richard Burke, an onsite spokesman for the recovery operation. "They don't get to come back and look at pictures. They don't get to come back and drive down their street."
The mudslide also clogged the Stillaguamish River, which then cut a slow-moving channel through the mud and debris, allowing floodwaters in the area to drain as rains abated.
Search teams were hurrying to make more progress ahead of several days of rain forecast to begin on Thursday, Burke said.
"The rain is going to change the dynamic about what happens out here," he said.
As well as making the ground surface more soggy and difficult to navigate, heavy rains increase the risk of further slides, flash flooding and greater exposure to toxic chemicals mixed in with the soil and water.
Burke said it was inevitable that some contaminants would seep out of the site and downstream into the river.
Search efforts in the weeks ahead could be further hampered by runoff from melting snow in the Cascades that is expected to pour through the narrow, newly created stream channel along the Stillaguamish River, possibly flooding the mud-pile site.
Sections of the slide area, already under 25 feet of water and believed to contain more human remains, could be submerged by more than 100 feet of water within three to four weeks without a wider channel to divert the partially dammed river, said Mike Asher, an area fire chief acting as the head of operations for the east side of the disaster zone.
"There's a lot of snow left on the mountains surrounding the valley," Asher said. "We're going to start facing runoff issues from that in the very near future."
The U.S. Army Corps of Engineers was working on a plan to dig a river channel, likely to be accompanied by levies, to keep the muddy, contaminated disaster site cordoned off, Asher said.
If the diversion goes wrong, the river could flow west down Highway 530, flooding additional stretches of the road and homes alongside it, he said.
No sign of life has been detected among the missing since the day of the slide, when eight injured people were rescued.
Authorities say that accounting for the number of dead has been complicated by the fact that the bodies are not always found intact. They acknowledge that some victims might be forever entombed under the massive pile of mud and rubble.
President Barack Obama on Wednesday approved the state's request for a major disaster declaration for the slide, making federal relief assistance available to individuals, households and businesses impacted by the tragedy, said Governor Jay Inslee.
Twenty-six of those killed in the mudslide have been identified, and authorities are still working to identify the three remaining confirmed fatalities, the Snohomish County Medical Examiner's Office said in a statement. (Editing by Alex Dobuzinskis, Cynthia Johnston and Chris Gallagher)